SUMMARY A group of epidemiologically related strains of , which were untypable by standard methods, became sensitive to typing phage 187 after the inocula had been held at 56° for 2 min. The strains were found to be lysogenic. Evidence is presented to show that the phage carried, phage 5504′, was exerting interference against phage 187 which could be partially overcome by heat treatment. Phage 187 grown in heated cocci was found to have undergone a host-induced modification which rendered it capable of lysing unheated lysogenic organisms. Active multiplication of the carried phage, phage 5504′, appeared to be necessary for the multiplication of phage 187. Ultraviolet irradiation also increased the sensitivity of the strain to phage 187, but to a lesser degree than heat treatment. It is suggested that the effect of heating is to induce active multiplication of phage 5504′ in the lysogenic culture, possibly by destroying a cytoplasmic substance responsible for maintaining the carried phage in its prophage state.


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